Implement accuracy, precision and recall metrics for multiclass classification evaluation.

[przemekwitek]

Users should be able to evaluate multiclass classification analysis with a bunch of single-valued metrics that give overall indication of classifier performance.
The most well-known metrics are:

• accuracy
  Implement accuracy metric for multiclass classification #47772
• precision
  Implement precision and recall metrics for classification evaluation #49671
• recall
  Implement precision and recall metrics for classification evaluation #49671

[elasticmachine]

Pinging @elastic/ml-core (:ml)